Phosphorus (P) concentrations in soil solution are generally low, whereas those near fertilizer particles may reach much greater values. The process governing P sorption at these high concentrations is obscure. Thus, the sorption mechanisms at high P concentrations on a synthetic goethite were investigated to assess the influence of ionic strength on P sorption and release. In addition to classic isotherms, the adsorption was investigated by Fourier-transform infrared spectroscopy and electrophoretic mobility. At constant ionic strength, the maximum amount adsorbed was about 2.3 μmol P m−2, which remained constant even at the highest concentrations. With variable ionic strength, the sorption isotherm showed plateaus at 2.3 and 5.3 μmol P m−2. Zeta potential and particle size data indicated that the increased ionic strength generated by P addition caused a compression of the phosphate ion swarm closer to the oxide surface, resulting in a reduction of the electrostatic repulsive forces and leading to an increase in sorption. The results of P release and Fourier-transform infrared spectra of the goethite-P complexes confirm this hypothesis. Thus, at high concentrations, sorption is possible beyond the capacity of the goethite singly coordinated surface groups, suggesting that both chemisorption and physical mechanisms are involved.